Corrosion is a problem which leads to high maintenance and repair overheads in many different industries. Prompt detection of problems caused by corrosion is necessary in order for effective mitigation strategies to be put in place. Various different methods of detecting corrosion in a metallic material have been developed. Corrosion sensors have been developed that are able, for example, to detect corrosion, or to monitor the progress of corrosion, or to monitor the degradation of protective layers applied to corrosive surfaces.
A simple method to monitor corrosion is the visual inspection of a sample of the material of interest. Other sensors have been developed that are more appropriate for the through life monitoring of a structure. Such sensors can be readily interrogated to provide data relating to the Many of these known corrosion sensors rely on electrical measurements performed on a thin film of metallic material to determine the level of corrosion that has occurred to a structure. Two known types of corrosion sensor are described in the Applicant's published European Patent Applications, Publication Numbers EP1554563 and EP1546679. These prior-known sensors comprise patterned conductive thin films formed on a substrate. The film, which is made of a material that mimics the characteristics of the bulk material to which the sensor is attached, defines a plurality of serpentine tracks extending between common terminals. These sensors can be used as resistive sensors, in which case the resistance of the sensors is measured over a period of time. The result of the action of corrosive media on the tracks is an increase in the overall resistance of the sensor, as measured between the common terminals. This measured increase in resistance can then be related to the effects of corrosion acting on the bulk structure to which the sensor is attached.
A problem that exists in sensors of the above type is that it can be difficult to relate the sensitivity of the sensor to corrosive media to the sensitivity of the bulk structure to corrosive media. Where a sensor is used to monitor the progress of corrosion to a bulk structure, it is important that the sensor corrodes at a rate at least approximately equal to that at which the bulk structure corrodes; or alternatively that the rate at which the sensor material corrodes can be easily related to the rate at which the bulk structure corrodes. Whilst, in the above-referenced applications, it has been disclosed to configure the tracks in order to reduce geometric effects on the rate of corrosion of the tracks, it remains necessary to ensure that the thin film material closely mimics that of the bulk structure to which the sensor is attached. This is done firstly by using, for the thin film tracks, metallic material having the same composition as the alloy or metal from which the bulk structure is fabricated, and secondly by annealing the thin film in order to ensure that the microstructure of the thin film is at least approximately the same as that of the alloy or metal from which the bulk structure is fabricated.
An alternative corrosion sensing strategy is proposed in the paper “Corrosion Sensors in Platform Management” by D. G. Dixon, M. C. Hebbron, S. J. Harris and A. Rezai and presented at the 1st World Congress on Corrosion in the Military, 1st June 2005. The authors refer to the above-referenced published patent applications, and propose a similar resistive sensor. However, the sensor disclosed by Dixon et al. is covered with a corrosion-inhibiting primer paint that is provided with an intentional defect in order to mimic, for example, the effect of a crack. The sensor is located on the structure to be monitored. At first, the defect will be protected from corrosive media by inhibitor species leaching from the paint, but, once the reservoir of inhibitor is exhausted, the sensor tracks will corrode, and a corresponding increase in the sensor resistance can be measured. Corrective action can then be taken. Such sensors are also known as “Inhibitor Depletion Sensors”.
In the case of an inhibitor depletion sensor, it is important that the resistive tracks beneath the defects corrode rapidly once the provisional protective effect of the inhibitor leaching from the paint has been exhausted, so that an operator can be made aware as quickly as possible that maintenance of the structure may be necessary. Thus there exists, in both cases, a need to be able to tailor the sensitivity of any particular type of corrosion sensor. The present invention arose as a result of consideration of the above-identified problems.